Interview preparation guide · Updated 2026
JavaScript Interview Questions with Answers and Code (2026)
A strong JavaScript interview answer connects language behavior to a concrete example. You should be able to reason about scope, closures, prototypes, coercion, asynchronous execution, and common data transformations, then write a small implementation and test its edge cases. Clear reasoning matters more than recalling obscure trivia.
By FrontendPro Editorial Team Last reviewed August 8, 2026
What you need to know
Use these topic clusters as a checklist. Each answer should define the concept, show where it matters, and include a concrete example or trade-off.
Scope, closures, and hoistingTrace where variables are declared, when bindings are created, and which values a function retains after its outer function returns.The event loop and asynchronous JavaScriptExplain the call stack, task and microtask queues, promises, timers, async functions, and the order in which observable output appears.Objects, prototypes, and classesUnderstand property lookup, prototype delegation, constructors, class syntax, method binding, and how inheritance works at runtime.Types, equality, and coercionKnow JavaScript primitives, reference values, strict equality, truthiness, nullish behavior, and the conversions that operators perform.Functions and common polyfillsPractice debounce, throttle, memoization, Promise utilities, event emitters, and array-method implementations with meaningful test cases.Output-based reasoningPredict output by tracing execution step by step rather than guessing, especially when closures, mutation, promises, or coercion interact.
Practice questions with answers
Open a question for a direct explanation, detailed answer, related concepts, and runnable code where the problem includes a coding exercise.
JavaScript interview questions
- Write mergeSorted(a, b) merging two sorted arrays into one sorted array, without sort()Advanced · Answer and explanation
- Write mostFrequentChar(str) returning the most frequent character (null for an empty string)Intermediate · Answer and explanation
- Write isAnagram(a, b) checking whether two strings are anagrams (ignoring case and spaces)Intermediate · Answer and explanation
- Write twoSum(nums, target) returning the indices of the two numbers adding up to target (empty array if none)Intermediate · Answer and explanation
- Write groupByLength(words) that groups an array of strings into an object keyed by lengthIntermediate · Answer and explanation
- Write flatten(arr) that flattens a nested array at any depthIntermediate · Answer and explanation
- Write chunk(arr, size) that splits an array into groups of sizeIntermediate · Answer and explanation
- Write titleCase(str) that uppercases the first letter of every word and lowercases the restBasic · Answer and explanation
- Write fizzBuzz(n) returning the FizzBuzz sequence from 1 to n as an array of stringsBasic · Answer and explanation
- Write countVowels(str) that counts the vowels in a stringBasic · Answer and explanation
- Write a function that removes duplicate values from an arrayIntermediate · Answer and explanation
- Write a function that checks whether a string is a palindromeBasic · Answer and explanation
- Write a function that finds the largest number in an arrayBasic · Answer and explanation
- Write a function that sums the numbers in an arrayBasic · Answer and explanation
- Write a function that reverses a stringBasic · Answer and explanation
- When would you use document.write()?Advanced · Answer and explanation
- Is TypeScript worth the trade-offs?Advanced · Answer and explanation
- How do you organise a codebase?Intermediate · Answer and explanation
- How do you share code between files?Basic · Answer and explanation
- How do you make an SPA SEO-friendly?Intermediate · Answer and explanation
- What is the same-origin policy?Intermediate · Answer and explanation
- How do you store auth tokens securely?Advanced · Answer and explanation
- How do you find security vulnerabilities?Intermediate · Answer and explanation
- Why does input validation matter for security?Intermediate · Answer and explanation
- How do you prevent clickjacking?Advanced · Answer and explanation
- Which security headers should you set?Intermediate · Answer and explanation
- What is Content Security Policy (CSP)?Intermediate · Answer and explanation
- How do you handle sensitive data?Intermediate · Answer and explanation
- How do you prevent SQL injection?Intermediate · Answer and explanation
- What is CSRF and how do you prevent it?Intermediate · Answer and explanation
How to prepare efficiently
Do not memorize a long script. Practice a repeatable answer structure: direct answer, short example, edge case, and trade-off.
- State the relevant JavaScript rule first, then walk through a minimal code example.
- For output questions, write down the call stack and queue transitions in execution order.
- For implementation questions, clarify inputs, outputs, timing behavior, and edge cases before coding.
- Test empty values, repeated calls, error paths, and boundary conditions instead of only the happy path.
- Compare related concepts such as debounce versus throttle or microtasks versus tasks.